Even when सर्व comes with दिशावाची बहुव्रीहि समास the सर्वनाम status is retain (still applies). Note that if this doesn't happen then द्वन्द्व would have the pluralization as well (causing ambiguity).
For example:
उत्तरस्य पूर्वस्याश्च दिशोरन्तरालम् => उत्तरपूर्वा ङे => उत्तरपूर्वस्य्ऐ (uttar and purva didn't combine to be treated as द्विवचन ; The द्विवचन would imply द्वन्द्व)